Darleyp

I attach an image of an 1857 map taken as a screenshot of two pages. There is major distortion along the spine of the book, where the pages join. This must be a rather common problem. Can anyone help me to resolve the distortion?

I am a beginner and starting self-tuition via tutorials. But I have lost the display that shows the Histogram, etc, and the layers of the image. Could you advise how I recover this?